The invention relates to a method and an apparatus for damping acoustic waves, that are to be damped, in a fluid that is disposed in a volume. The invention especially relates to damping using an apparatus that includes a cavity that is enclosed by delimiting surfaces, at least one of the delimiting surfaces being permeable to fluid, in order to enable fluid to be exchanged with the volume.
Acoustic waves, e.g. sound waves, occur at many locations in the environment. Materials and structures for acoustic damping that have the ability to weaken acoustic waves using energy dissipation are needed keep sound propagation in check in terms of noise protection and are also needed to reduce sound propagation in technical systems. This latter item can be important e.g. for mensuration applications in order for instance to produce a so-called low-reflection room or to suppress undesired technical phenomena that occur due to fluctuations in acoustic pressure. One very important example of the latter phenomenon are thermoacoustic vibrations in combustion chambers, in particular in combustion chambers in turbo-engines, such as for instance jet engines and gas turbines.
In many areas of applications, perforated surface linings are used for damping acoustic waves. They are generally embodied as flat double-walled structures, one wall being provided with apertures. These apertures open into a cavity that is disposed between the two walls or into a plurality of small cavities if there is a honeycomb-like structure between the walls. This arrangement is able to weaken acoustic waves, i.e. sound fields, on the side that is perforated.
Frequently there is an acoustic damping material, instead of the honeycomb-like structure, in apparatus for sound damping that are installed in buildings or that are used in noise protection walls. Such apparatus for sound damping are also used in fan engines and stationary gas turbines; in these cases they are generally called “liners”. Liners have a number of embodiments. They are employed for acoustic damping e.g. in combustion chambers upstream of or in the intake or bypass to engines.
In particular the very strong combustion fluctuations that are caused in particular by a feedback mechanism of combustion instabilities with acoustic resonances and that are primarily standing wave fields frequently occur in so-called low NO burners that are optimized for low nitrogen oxide emissions. Because of the high repetition rates for the acoustic cycles, when there is flow or combustion-induced excitation of the system in the vicinity of a natural frequency of the combustion chamber there is an excited resonance in the combustion chamber. It is characteristic of standing wave fields that are embodied in spatially delimited combustion chambers that they have very strong pressure fluctuation amplitudes e.g. at the combustion chamber closure or at the turbine intake. If the damping is inadequate, the loads that occur here at the combustion chamber wall can lead to deformation of the combustion chamber wall or, worst case, to the destruction of the combustion chamber.
EP 0 702 141 A2 for instance describes a wall arrangement for an exhaust gas nozzle in an ultrasonic jet turbine in which a sound-absorbing element that comprises a porous plate and a honeycomb structure or a porous plate and a torus core is arranged between a nozzle plate and a liner. Cooling air is caused to flow along an interior surface of the nozzle plate and to flow through apertures in the porous plate against the liner.
Known from EP 1 762 786 A1 is a method for damping thermoacoustic vibrations by means of a resonator that has a resonator volume, thermoacoustic vibrations being induced in a cavity that tends towards thermoacoustic vibrations and at least some of the thermoacoustic vibrations being removed from the cavity and added to the resonator volume, damping vibrations being produced in the resonator volume, which damping vibrations are tuned to the thermoacoustic vibrations, at least some of the damping vibrations being added to the cavity such that the damping vibrations and the thermoacoustic vibrations overlap in an overlap area, and the vibrations largely cancel one another in the overlap area. Furthermore described is an apparatus for damping thermoacoustic vibrations in the vibration-capable cavity, which apparatus includes a resonator having a resonator volume and a cavity such that the cavity is joined via a decoupling line to the resonator volume and in which the decoupling line has an adjusting device by means of which a thermoacoustic vibration in the decoupling line can be influenced and in which another resonator tube connects the resonator volume to the cavity. The cavity is preferably a combustion chamber, in particular that of a gas turbine. The resonator is a so-called Helmholtz resonator. The latter is distinguished in that the resonantly excited vibrations therein are excited or produced by the sound waves that are produced in the volume to be damped. Even if in the described apparatus a resonance frequency of the damping vibrations can be influenced via the adjusting device by a longitudinal displacement of the decoupling line, such a Helmholtz resonator can only weaken acoustic vibrations by the resonance frequency of the resonator in a very narrow frequency band.
Known from patent application DE 10 2005 059 184, not laid open, is an apparatus for damping thermoacoustic resonances in combustion chambers that includes a combustion chamber, at least one at least partially hollow cylindrical injection element that is coaxial with the combustion chamber being provided that has a plurality of nozzle apertures spaced apart from one another, and a fluid for dissipating vibration energy that occurs during the combustion process flowing into the combustion chamber through the nozzle openings as a free jet having at least one radial component. Thus DE 10 2005 059 184 describes that the vibrations can be damped when an element that is embodied partially as a hollow cylinder and that surrounds the cylindrical combustion chamber has, on a limiting surface of the combustion chamber, nozzle openings through which a fluid is caused to flow in. In order to use this effect, however, some of the fluid that was compressed by the turbo-engine must be used and is thus no longer available for other processes, such as combustion or increasing thrust. The method described therein is therefore always associated with a certain loss of energy that is not negligible. The method thus significantly reduces the efficiency of the machine.
Nor do other applications in the field of noise protection outdoors and in buildings provide adequate acoustic damping. Acceptance in the general population of techniques and technologies that primarily produce noise cannot be increased unless noise propagation can be limited using improved methods and apparatus for damping.